If you want solar photovoltaic panels on your home but don’t have the money for them, EDF Energy is offering to install a free system – complete with storage batteries – if you agree to buy the subsidised power it generates for 20 years.

The French-owned energy giant is looking for 100 homes to trial its Sunplug scheme, which is being offered in conjunction with established solar supplier Lightsource.

To sign up you need to have a large, unshaded south-facing roof at a pitch of about 45 degrees. If you are accepted, the company will install the largest solar panel system the roof can take – a 16-panel setup will generate 4kW – plus an LG storage battery that lets you use the power that’s generated during the day in the evening.

In return, EDF gets to keep the feed-in tariff paid by the government, which is worth about £150 a year. It also keeps the export tariff – around £50 a year. The householder is contractually bound to pay Sunplug 9.9p per kilowatt hour for each unit of electricity they use from the panels and battery. This is a little cheaper than what you would pay if you bought green electricity from the grid. For example, green supplier Good Energy charges 15.5p, with a standing charge averaging 26p a day.

The advantage could come in future years as the price demanded by Sunplug can only rise by the retail prices index or 2.5% – whichever is lower. If the price of grid electricity rises substantially over the next 20 years, users will make considerable savings.

However, if they don’t, some users will be left wondering why they bothered, not least because they have to have the system inspected each year, which will cost about £80. So this scheme is likely to appeal to anyone who wants green electricity at fixed prices over the next two decades.

The other significant benefit comes at the end of the 20-year term, when the householder is given ownership of the system, which should continue to generate substantial free power.

Robin Melvin, head of innovation at EDF, says about 50 households have signed up for the trial and that he is looking for another 50 to join, at which point its success will be evaluated.

He says users should not have a problem with their mortgage company – previously an issue with free solar installations – as the package is compliant with Council of Mortgage Lenders guidelines and big lenders are on board with the deal. The top-up electricity most households will need can be bought from any supplier at normal rates.

If you decide to move house during the 20-year term, he says the deal can be passed on to the new owner, or the seller can buy the system out. The prices are pre-agreed on a sliding scale – it will cost £5,500 to exit the initiative after 10 years.

If you like the idea of the scheme and have the money to buy it outright – probably the best option – Sunplug will sell you a 3kW (12 panels) system for £6,999 and a 4kW (16 panels) one for £7,499. You get to keep the feed-in tariffs and export payments, benefit from the electricity savings and also get the crucial battery.

In-house storage batteries are seen as the next big thing in renewable energy and are increasingly being offered by suppliers alongside electricity generating panels as they allow households to use double the power they produce – they can access it when they need it in the evenings.

Last week, Ikea announced it is going to sell solar PV/battery installations, in conjunction with Solarcentury, at similar prices. It is offering a system that generates 3.24kW from £6,925. It estimates that homeowners can pay off the capital invested in the system in roughly 12 years through feed-in tariffs and lower electricity bills, assuming they have the right kind of roof.

So what’s the Money verdict? Solar PV systems are still a good investment if you have the money upfront, the right roof and location, and if you plan to stay in the house for a long time. The case for the free Sunplug deal is less clear. To us, it looks too heavily weighted in favour of the company. If it offered some free electricity each day or other incentives, that would make the scheme more attractive.